1. Field of the Invention
The present inventions relate to an electric steering system for a small boat, and in particular, to steering systems that provide variable steering response.
2. Description of the Related Art
Japanese Patent No. JP-B-2739208 discloses an electric steering apparatus for watercraft using an electric motor for changing the steering angle of an outboard motor. The electric steering apparatus disclosed therein includes a sensor for detecting the rotational angle and direction of operation of a steering wheel, as well as a controller for controlling the electric motor of the steering device based on a detection signal from the sensor. The steering wheel is electronically connected to the electric motor via the sensor and the controller. This configuration allows steering of a boat by driving the electric motor according to an operation amount of the steering wheel.
With such an electric steering apparatus, however, no feedback force is provided to the operator. For example, when wind or other water currents exert loads on the outboard motor, the operator is unaware of any such loads. Thus, the operator cannot feel those external forces which can manifest themselves as a heavy or light feel of the steering wheel during operation. These external forces can be caused by waves, wind, etc. As such, the operator is less able to respond to such loads and thus is less able to quickly respond to such loads. More experienced operators of boats using cable-type steering connections can feel these loads and react quickly to keep the watercraft on a desired course.
Other forces also affect the steering of watercraft. For example, the rotation of a propeller in the vicinity of a rudder can create a hydrodynamic effect that generates a load against the rudder; the effect is known as a “paddle-rudder effect.” This effect can alter the course of a boat during operation.
The paddle-rudder effect is described in Japanese Patent No. JP-B-2959044, in which the paddle-rudder effect is referred to as a “gyro effect.” The paddle-rudder effect can cause a boat to proceed in a direction that is deflected to the left or to the right by a certain angle offset from the angle that would result from the rudder position if the boat were coasting with no rotation of the propeller. In other words, even where the steering wheel is set at a neutral position (a zero steering angle), the boat proceeds at an angle offset from a straight ahead direction.
The JP-B-2959044 patent discloses a technique for compensating for the paddle-rudder effect (gyro effect) by applying a predetermined torque to the rudder according to the steering wheel operation angle. In this technique, however, a constant torque is required to be applied via an electric motor in such a direction so as to cancel the paddle-rudder effect. This results in a continuous power consumption by the electric motor at all times.
Further, the technique disclosed in JP-B-2959044 does not compensate for the lack of feedback feeling in the steering wheel, nor does it describe a system that can provide reaction forces at the steering wheel. Thus, with the system and technique disclosed in the JP-B-2959044 patent, it is impossible to obtain an operating feeling depending on the steering wheel operation or a driving feeling through the steering wheel. Thus, it is difficult to recognize imminent course changes that may result from the waves, wind or the paddle-rudder effect and to quickly respond to these forces to maintain a desired course.
Japanese Patent Publication No. JP-A-HEI 10-226346 discloses an electric steering system for an automobile. In this system, a reaction force motor applies a reaction force to the steering wheel of the automobile to communicate reaction forces exerted from the ground equally through the left and right tires of the automobile. This control technique for automobiles is not simply applicable to boats, which receive a force due to the paddle-rudder effect described above, as well as other effects.